2019 GSICS Data and Research Working Groups Annual Meeting Review of atmospheric radiative transfer models suitable for vicarious calibration 2019 GSICS Data and Research Working Groups Annual Meeting Yves Govaerts & Vincent Leroy Rayference ESRIN, Frascati, 4 – 8 March 2019

Overview Radiative transfer models play a critical role for vicarious calibration; Instruments like S2/MSI have proven to have a radiometric accuracy close to 2-3%; What is the expected simulation accuracy with current 1D RTMs over CEOS calibration sites: 1%, 3% or 5%? RTM numerical accuracy Input parameter accuracy Target of choice: water – appears dark in all 8 bands Sentinel 2 doesn’t make observations over open waters Open waters may appear too dark Coastal waters may appear brighter due to the presence of suspended matter 2

Physics is based on two fundamental pillars Concept Physics is based on two fundamental pillars OBSERVATION THEORY

Physics is based on two fundamental pillars Concept Physics is based on two fundamental pillars OBSERVATION THEORY MEASUREMENT DEVICE MATHEMATICAL MODEL

Concept Physics is based on two fundamental pillars OBSERVATION THEORY RADIATIVE TRANSFER MODEL SATELLITE RADIOMETER

Experimental Setup Acquisition of TOA BRF over Libya-4 acquired by (PICSCAR) Envisat/MERIS AQUA/MODIS S2A/MSI L8/OLI Simulation of these TOA BRFs with 3 different RTMs fed with the same surface and aerosol properties (Govaerts et al., 2004, 2013) Estimation of the mean bias between observation and simulation Code RTE solver Gas Surface Aerosol 6S-V Successive order Hitran96 RPV 4P Customised LibradtranV2 Monte Carlo Hitran2012 RTMOM Matrix Operator

S2A/MSI All SZA values

S2A/MSI

S2A/MSI

S2A/MSI All SZA values

L8/OLI All SZA values

S2A/MSI All SZA values

L8/OLI

MERIS 412 band ZSA < 30 SIXS-V RTMOM Libradtran

Typical RTM functionalities Man-Machine Interface Collects the input information concerning the Spectral range and resolution; Atmospheric vertical composition; Cloud and aerosol concentration; Surface properties; Type of “measurements” Up/down Flux, radiance reflectance transmittance Elevation 06/03/2019

Typical RTM functionalities Driver Converts the input information into data that can be understood by the RTE solver at a given wavelength, i.e. for each atmospheric layer: The phase function The single scattering albedo The molecular transmittances It provides the lower boundary conditions. It also performs the vertical rescaling of molecular concentration if needed. 06/03/2019

Typical RTM functionalities Driver Converts the input information into data that can be understood by the RTE solver at a given wavelength, i.e. for each atmospheric layer: The phase function The single scattering albedo The molecular transmittances The lower boundary conditions (if needed) It also performs the vertical rescaling of molecular concentration if needed. RTMs without a versatile Driver should not be considered for operational vicarious calibration because there are too cumbersome to operate. 06/03/2019

Typical RTM functionalities RTE Solver Solves the radiative transfer equation in each scattering element (e.g. atmospheric layers) with a specific numerical methods: Successive Order of Scattering Discrete ordinate Adding/doubling Matrix Operator Spherical harmonic Monte Carlo 06/03/2019

Review of existing models 1D plane parallel atmosphere Vertical structure of the atmosphere No 3D cloud effects (e.g. for DCC) Only flat surface Not accurate for large sun and viewing angles because of the plane parallel approximation. 3D plane parallel atmosphere The atmosphere is divided into regular voxels Each voxel might have different optical properties RTE solver : discrete ordinate or Monte Carlo 06/03/2019

Toward a 1% RTM accuracy Surface BRF : accounting for topography (e.g., oriented sand dunes); Molecular absorption: account for species like O4; Rigorous calculation of the coupling between: Surface reflectance and atmosphere scattering; Aerosol scattering and molecular absorption; Polarization, non flat earth for large zenith angles; Improvement of the surface and atmospheric property characterization; 06/03/2019

The Eradiate RTM New open source 3D RTM specifically dedicated to Cal/Val activities; Based on most advanced 3D Monte Carlo Ray Tracing rendering techniques; Not limited to only one (atmospheric) community; Will include 3D representation of land / ocean / atmospheric / cryosphere in a single framework; Will allow the simulation of BRF field at the infinity; Satellite images; Ground observations.

Development Phases 06/03/2019

Phase 1 : Planned Scene Elements 1D Atmosphere Plane-parallel (“flat-Earth”) Layered spheroids (“round-Earth”) Surface Standard empirical BRF models (e.g. RPV, Ross-Li, Hapke) Microfacet models (e.g. semi-discrete, Oren-Nayar, Torrance-Sparrow, Cox-Munk) Including parameter texturing 3D scenes with detailed typography and objects (e.g. Libya-4, RadCalNet, Dome-C, …) Illumination Infinitely distant collimated Finite-size solar disc (uncollimated) Sensors Flux & radiance meters Ideal detector (pinhole camera) BRF at finite or infinite distance 06/03/2019 Eradiate Status & Upcoming Actions

www.eradiate.eu The Eradiate RTM Please register to the Eradiate newsletter (under contact) to be updated on Eradiate latest developments. Yearly Eradiate workshops are organised at the JRC, Ispra site (IT). The next (third) one will take place on Nov 26th - 27th 2019 (save the date!). Our sponsors (for this presentation)